Led utilizing internal color conversion with light extraction enhancements

ABSTRACT

A light emitting diode (LED) device may include an n-type layer formed on a transparent substrate. A photoluminescent (PL) in the n-type layer quantum well (QW) and an electroluminescent (EL) QW may be formed on the n-type layer. The PL QW and the EL QW may be separated from one another by a portion of the n-type layer. A p-type layer may be formed on the EL QW. Trenches may be formed extending into the n-type layer, the trenches defining an emitting area. A passivation material may be formed on sidewalls of the trenches and n-type contacts may be formed therein. A p-type contact may be formed on an upper surface of the p-type layer. A dichroic mirror may be formed on at least a lower surface of the transparent substrate.

BACKGROUND

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes (VCSELs), and edge emitting lasers are among the mostefficient light sources currently available. Materials systems currentlyof interest in the manufacture of high-brightness light emitting devicescapable of operation across the visible spectrum include Group III-Vsemiconductors, particularly binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and nitrogen, also referred to as III-nitridematerials.

Typically, III-nitride light emitting devices are fabricated byepitaxially growing a stack of semiconductor layers of differentcompositions and dopant concentrations on a sapphire, silicon carbide,III-nitride, or other suitable substrate by metal-organic chemical vapordeposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxialtechniques. The stack often includes one or more n-type layers dopedwith, for example, silicon, formed over the substrate, one or more lightemitting layers in an active region (e.g., a p-n diode) formed over then-type layer or layers, and one or more p-type layers doped with, forexample, magnesium, formed over the active region. Electrical contactsare formed on the n-type and p-type regions.

One class of blue and green LEDs use GaInN/GaN strained quantum wells orGaInN/GaInN strained quantum wells located between the n-type and p-typelayers to generate light by the recombination of holes and electronsinjected from these layers. The present disclosure generally relates toimprove the efficiency of these quantum well devices.

SUMMARY

A light emitting diode (LED) device may include an n-type layer formedon a transparent substrate. A photoluminescent (PL) in the n-type layerquantum well (QW) and an electroluminescent (EL) QW may be formed on then-type layer. The PL QW and the EL QW may be separated from one anotherby a portion of the n-type layer. A p-type layer may be formed on the ELQW. Trenches may be formed extending into the n-type layer, the trenchesdefining an emitting area. A passivation material may be formed onsidewalls of the trenches and n-type contacts may be formed therein. Ap-type contact may be formed on an upper surface of the p-type layer. Adichroic mirror may be formed on at least a lower surface of thetransparent substrate.

A LED may include a substrate and a first epitaxial layer formed on thesubstrate. An epitaxial reflector may be formed on the first epitaxiallayer. The epitaxial reflector may include multiple layers of GroupIII-V semiconductor materials having different compositions. A secondepitaxial layer may be formed on the epitaxial reflector. An n-typelayer may be formed on the second epitaxial layer. A photoluminescent PLQW may be formed in the n-type layer. An EL QW may be formed on then-type layer. The EL QW and the PL QW may be separated from one anotherby a portion of the n-type layer. A p-type layer may be formed on the ELQW. A p-type electrode may be formed on an upper surface of the p-typelayer. A dielectric passivation layer may be formed on the upper surfaceof the p-type layer, sidewalls of the p-type layer, sidewalls of the ELQW, sidewalls of the portion of the n-type layer, sidewalls of PL QW. Ann-type electrode may be formed on the dielectric passivation layer andthe n-type layer.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding may be had from the following description,given by way of example in conjunction with the accompanying drawings,wherein like reference numerals in the figures indicate like elements,and wherein:

FIG. 1 is a graph illustrating the emission of a light emitting diode(LED) wafer utilizing electroluminescence (EL) and photoluminescence(PL) in which the EL is insufficiently absorbed by PL emitting quantumwells, resulting in an undesirable double-peaked spectrum;

FIG. 2A is a conventional green LED;

FIG. 2B is a green LED utilizing PL obtained by conversion of violet EL;

FIG. 2C is a white LED wafer utilizing blue EL and partial conversion ofblue EL into green PL and red PL;

FIG. 3 is a cross-section view illustrating a green LED with anepitaxial reflector;

FIG. 4A is a cross-section view illustrating patterning and etching theepitaxial layers shown in FIG. 3 to expose an upper surface of an n-typeepitaxial layer;

FIG. 4B illustrates forming a reflecting p-electrode on an upper surfaceof a p-type layer;

FIG. 4C illustrates forming a conformal dielectric passivation layer onan upper surface and sides of the p-type layer, sides of an EL QW, sidesof an n-type layer, sides of a PL QW, and an upper surface of theundoped epitaxial layer;

FIG. 4D illustrates forming a reflecting n-type electrode on thedielectric passivation layer;

FIGS. 4E-4F illustrate the reflectivity of the epitaxial reflector basedon wavelength and incident angle;

FIG. 5 is a chart illustrating reflectivity of the epitaxial reflectorover different angles of incidence for different wavelengths of light;

FIG. 6A is a cross-section view illustrating the use of anelectrochemical reaction used to improve the reflectivity of theepitaxial reflector;

FIG. 6B is a cross-section view illustrating a reaction that mayselectively oxidize nitride layers of higher Al mole fraction and mayconvert them into oxide or oxy-nitride layers of lower refractive indexthan as-grown material;

FIG. 6C is a cross-section view illustrating a reaction that introducesmicroscopic voids (i.e., porosity) into layers that are highly dopedwith Si or Ge;

FIG. 7A is a cross section view illustrating forming trenches in thegreen LED utilizing PL of FIG. 2B to define an emitting region;

FIG. 7B is a cross section view illustrating forming a conformaldielectric passivation layer in the trenches and on the p-type layer;

FIG. 7C is a cross section view illustrating forming n-type contacts inthe trenches and a p-type contact on the p-type layer;

FIG. 7D is a cross section view illustrating forming a dichroic mirroron a bottom surface of the LED device;

FIG. 7E is a cross section view illustrating forming a dichroic mirroron a bottom surface and sides of the LED device;

FIG. 8A is a cross section view illustrating forming trenches in thegreen LED utilizing PL of FIG. 2B to define an emitting region;

FIG. 8B is a cross section view illustrating forming a conformaldielectric passivation layer in the trenches and on the p-type layer;

FIG. 8C is a cross section view illustrating forming n-type contacts inthe trenches and a p-type contact on the p-type layer;

FIG. 8D is a cross section view illustrating forming a dichroic mirroron a bottom surface of the LED device;

FIG. 8E is a cross section view illustrating forming a dichroic mirroron a bottom surface and sides of the LED device;

FIGS. 9A-9B are charts illustrating reflectivity of two differentdichroic mirror designs over different angles of incidence for differentwavelengths of light; and

FIG. 10 is a chart illustrating power emitted as a function of distancebetween the p-type contact and the EL QW.

DETAILED DESCRIPTION

Examples of different light emitting diode (“LED”) implementations willbe described more fully hereinafter with reference to the accompanyingdrawings. These examples are not mutually exclusive, and features foundin one example can be combined with features found in one or more otherexamples to achieve additional implementations. Accordingly, it will beunderstood that the examples shown in the accompanying drawings areprovided for illustrative purposes only and they are not intended tolimit the disclosure in any way. Like numbers refer to like elementsthroughout.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present invention. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present. Itwill also be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. It will be understood that these terms areintended to encompass different orientations of the element in additionto any orientation depicted in the figures.

Relative terms such as “below” or “above” or “upper” or “lower” or“horizontal” or “vertical” may be used herein to describe a relationshipof one element, layer or region to another element, layer or region asillustrated in the figures. It will be understood that these terms areintended to encompass different orientations of the device in additionto the orientation depicted in the figures.

There is interest in green and yellow LEDs with high radiance and highwall plug efficiency (WPE) for diverse applications such as displays(e.g., micro-displays), architectural lighting, and general illuminationsystems based on mixing the emission of direct color LEDs rather thanphosphor conversion. These applications may be limited by the relativelypoor WPE of conventional green and yellow LEDs. The so-called efficiencydroop phenomenon may be much more severe in conventional green LEDs ascompared to blue LEDs. Green LEDs may be particularly inefficient whenthey are driven at the high current densities required for projectiondisplay applications. The higher operating voltage of green LEDsrelative to blue and red LEDs further complicates the design of drivercircuitry and heat sinks.

The photoluminescence (PL) of green InGaN multi-quantum wells (MQWs)excited by absorption of shorter wavelength photons may be moreefficient than the electroluminescence (EL) excited by electricalinjection the same MQWs sandwiched in a p-n junction. This may beexplained at least in part by a more even distribution of carriersbetween the MQWs when carriers are generated by optical absorptioninstead of electrical injection. The efficiency droop in EL applicationsmay be exacerbated by an uneven distribution of carriers among the MQWsresulting from differences in the electrical transport behavior of holesand electrons. Using the PL from green MQWs excited by absorption of theEL of a shorter wavelength may be a promising method to improve theefficiency of high-radiance green LEDs. This concept may also benefitfrom the typically lower operating voltage of blue or near-ultravioletLEDs compared to state-of-the-art electrically-injected green LEDs.

Green emitting MQWs may be produced in a separate epitaxial growth runfrom blue quantum wells that emit the EL required to photo-pump thegreen MQWs. However, using separate growth runs may be undesirable withrespect to manufacturing throughput in the epitaxy process. In addition,multiple epitaxial growth runs may have the further drawback ofrequiring additional downstream manufacturing steps to join together theEL and PL components of the device in a way that efficiently coupleslight from one into the other. This approach may be impractical forapplications that require small form factors such as micro-LED displays.

It may be desirable to integrate the EL and PL components of the LED ina single epitaxial growth run. The green MQWs may be located, forexample, on an n-type side of a p-n junction, and may be separated fromelectrically injected shorter-wavelength MQWs by an n-type conductinglayer. As such, any p-n junction recombination current may not flowthrough the green MQWs.

As shown in FIG. 1, a diagram illustrating wavelength peaks of lightemitted from MQWs combining violet EL and green PL. Only a fraction ofviolet EL light (e.g., 400 nm) may be absorbed in the green MQWs. Alarge fraction of the 400 nm EL may escape from the LED without beingconverted to green. This may result in a double-peaked emission spectrumthat is not perceived by the eye as green.

The low single-pass absorption probability in a quantum well may need tobe overcome to use the PL concept in the manufacture of practical LEDswith useful color characteristics. More than 40% of blue light may betransmitted after passing once through a stack of 30 PL QWs. Many passesmay be required through a practical number of PL QWs to convert all ofthe blue light into green light.

The following description includes LED design improvements that mayhinder the escape of shorter-wavelength EL photons from a device whilepromoting the escape of longer-wavelength PL photons. These improvementsmay increase the probability that the shorter-wavelength EL will beinternally absorbed within the LED and converted to the desired longerwavelength while minimizing penalties in the extraction efficiency ofthe longer wavelength. The improvements described below may be used toproduce an LED of a single color (e.g., green or yellow) in which theextraction of the EL wavelength may be completely suppressed. Theimprovements may also be used to produce a phosphor-free white LED(e.g., using green and red PL emitting quantum wells) in which theextraction of the blue EL wavelength may be only partly suppressed.

As described in additional detail below, an epitaxial wafer may containa p-type layer, an n-type layer, and a first set of one or more QWs withshorter emission wavelength disposed between the n-type and p-typelayers. One or more additional sets of one or more QWs with longeremission wavelengths may be disposed on either side of, but not within,the depletion region between the n-type and p-type layers. The one ormore additional sets of QWs may be disposed on the n-type side as shownin FIGS. 3 and 4. The elements described above may be grown on the samesubstrate wafer in the same epitaxial growth run.

The longer wavelength QWs may have a peak wavelength at least 20 nmlonger and as much as 1200 nm longer than the shorter wavelength QWs.

An electrode may cover either the p-type or n-type layer. The electrodemay have high reflectivity for the EL emission wavelength, but notnecessarily for other wavelengths. The reflecting electrode may belocated on the p-type layer.

One or more of the following elements may be included to increase theprobability that shorter wavelength photons generated by EL may beabsorbed in the longer wavelength QWs.

A dichroic mirror may be coated on one or more external surfaces of theLED chip. The dichroic mirror may have a high reflectivity at theshorter wavelength of EL emissions and a low reflectivity at the longerwavelength of PL emissions over a wide range of angles of incidence.

A photonic crystal may be patterned into one or more external orinternal surfaces of the LED. The periodicity of the photonic crystalmay be selected to minimize diffraction of the shorter wavelength ELemissions and maximize diffraction of the longer wavelength PLemissions.

An epitaxial mirror may be grown within the epitaxial layer structure ofthe LED wafer. This epitaxial mirror may have a higher angle-averagedreflectivity for the shorter wavelength EL emissions as compared to thelonger-wavelength PL emissions.

A distributed Bragg reflector (DBR) may be integrated into the wafer.The DBR may be formed by the growth of a sequence of epitaxial layerswith differences in doping and/or alloy composition combined with apost-growth electrochemical reaction. The post-growth reaction may beselective with respect to doping and/or alloy composition and may reducethe effective refractive index of some of the layers in the epitaxialsequence. The thickness of the epitaxial layers may be chosen to resultin a DBR periodicity that maximizes reflectivity corresponding to theshorter wavelength of EL emissions.

The distance between the reflecting electrode described above and the ELemitting QWs may be selected to control the internal radiation angulardistribution of the EL emissions in a way that maximizes its absorptionin the PL emitting QWs.

Referring now to FIGS. 2A-2C, cross-section views of epitaxial wafersare shown. FIG. 2A illustrates a conventional green LED. FIG. 2Billustrates a green LED utilizing PL obtained by conversion of violetEL. FIG. 2C illustrates a white LED wafer utilizing blue EL and partialconversion of the blue EL into green PL and red PL.

FIGS. 2A-2C may have common features, such as a substrate 202, anunderlying n-type layer 204, a PL QW 206, and a p-type layer 208. Thesubstrate 202 may comprise a crystalline material and may be acommercial substrate. The substrate 202 may comprise sapphire, SiC, orGaN.

The n-type layer 204 may comprise any Group III-V semiconductors,including binary, ternary, and quaternary alloys of gallium, aluminum,indium, and nitrogen, also referred to as III-nitride materials. In anexample, the n-type layer 204 may comprise GaN. The n-type layer 204 maybe doped with n-type dopants, such as Si or Ge. The n-type layer 204 mayhave a dopant concentration significant enough to carry an electriccurrent laterally through the n-type layer 204. In an example, then-type layer 204 may be highly doped.

The n-type layer 204 may be formed using conventional depositiontechniques, such as metal-organic chemical vapor deposition (MOCVD),molecular beam epitaxy (MBE), or other epitaxial techniques. In anepitaxial deposition process, chemical reactants provided by one or moresource gases are controlled and the system parameters are set so thatdepositing atoms arrive at a deposition surface with sufficient energyto move around on the surface and orient themselves to the crystalarrangement of the atoms of the deposition surface. Accordingly, then-type layer 204 may be grown on the sapphire substrate 202 usingconventional epitaxial techniques. A nucleation layer (not shown) may beformed on the substrate 202 prior to the n-type layer 204. Thenucleation layer may comprise GaN or AlN.

The p-type layer 208 may be formed using the conventional epitaxialdeposition techniques described above. The p-type layer 208 may compriseany Group III-V semiconductors, including binary, ternary, andquaternary alloys of gallium, aluminum, indium, and nitrogen, alsoreferred to as III-nitride materials. In an example, the p-type layer208 may comprise GaN. The p-type layer 208 may be doped with p-typedopants, such as Mg. An electron blocking layer (not shown) may beformed below the p-type layer 208.

The PL QW 206 may be formed using the conventional epitaxial depositiontechniques described above. The PL QW 206 may comprise a sequence ofmultiple QWs emitting the same wavelength of light. The PL QW 206 maycomprise different layers of InGaN and GaN. In an example, the PL QW 206may emit a green light having a wavelength of approximately 530 nm. Theemission color may be controlled by the relative mole fractions of Inand Ga in the InGaN layer and/or thicknesses of the multiple QWs andbarrier thickness. A higher mole fraction of In may result in a longerwavelength.

An individual QW within the PL QW 206 may have an InGaN thicknessranging from approximately 0.5 nm to 15 nm and a GaN thickness rangingfrom approximately 2 nm to 100 nm. The total number of quantum wells inthe PL QW 206 may be between 1 and 50. The PL QW 206 may be located inthe n-type layer 204, near to, but not within, the depletion region ofthe p-n junction at operating forward bias.

As shown in FIG. 2B, the green LED utilizing a PL QW 206 may alsoinclude an EL QW 210 within a depletion region of the p-n junction. TheEL QW 210 may be formed using the conventional epitaxial depositiontechniques described above. The EL QW 210 may comprise a sequence ofmultiple QWs emitting the same wavelength of light. The EL QW 210 maycomprise different layers of InGaN and GaN. In an example, the EL QW 210may emit a violet light having a wavelength of approximately 400 nm.

The EL QW 210 may be separated from the PL QW 206 by a first distance D1of the n-type layer 204. The first distance D1 may range fromapproximately 5 nm to approximately 1000 nm. It should be noted, thefirst distance D₁ may comprise an additional layer of n-type materialgrown at temperature low enough to not degrade the optical properties ofthe layers beneath it.

As shown in FIG. 2C, a white LED may utilize one or more EL QWs andmultiple groups of PL QWs. The white LED may include the PL QW 206, asecond PL QW 212, and an EL QW 214. The second PL QW 212 may comprisedifferent layers of InGaN and GaN. The second PL QW 212 may comprise asequence of multiple QWs emitting the same wavelength of light. The ELQW 214 may comprise different layers of InGaN and GaN. The EL QW 214 maycomprise a sequence of multiple QWs emitting the same wavelength oflight.

The PL QW 206 and the second PL QW 212 may be located within the n-typelayer 204 near to, but not within, the depletion region of the p-njunction at operating forward bias. The EL QW 214 may be located withina depletion region of the p-n junction.

The second PL QW 212 and the EL QW 214 may be formed using theconventional epitaxial deposition techniques described above. The secondPL QW 212 and the EL QW 214 may comprise InGaN/GaN. In an example, thesecond PL QW 212 may emit a red light having a wavelength ofapproximately 610 nm. In an example, the EL QW 214 may emit a blue lighthaving a wavelength of approximately 440 nm. The EL QW 214 may beseparated from the second PL QW 210 by a second distance D₂ of then-type layer 204. The second PL QW 212 may be separated from the PL QW206 by a third distance D₃ of the n-type layer 204. The second distanceD₂ may range from approximately 5 nm to approximately 1000 nm. The thirddistance D₃ may range from approximately 5 nm to approximately 1000 nm.It should be noted that the second distance D₂ and the third distance D₃may comprise additional layers of n-type material grown at temperaturelow enough to not degrade the optical properties of the layers beneathit.

The optimum layer thickness, doping, and growth conditions for the PLQWs may not be the same as those parameters in an electrically injectedLED of the same color. The PL QWs and/or barriers thereof may be dopedwith donors such as Si or Ge in order to prevent any significant voltagedrop across the PL QW region when the p-n junction surrounding the ELactive region is in forward bias.

The rest of the growth process may proceed as for a conventional LEDwafer. The p-type layer 208 may have a thickness different from theoptimum p-type layer thickness in a conventional LED wafer. In an LEDwith a reflecting p-electrode, the thickness of the p-type layer 208 maybe adjusted to optimize the optical polarization state and internalradiation pattern of the EL emission for a particular purpose. Thethickness that maximizes internal conversion of EL to PL as describedherein may be different from the thickness that maximizes the lightoutput from a conventional LED.

Referring now to FIG. 3, a cross-section view illustrating a green LEDwith an epitaxial reflector 304 is shown. Additional epitaxial layersmay be grown for the purpose of selectively reflecting shorterwavelength light in a direction away from the substrate 202.

A first epitaxial layer 302 may be formed on the substrate 202 using oneor more of the epitaxial growth techniques described above. A nucleationlayer (not shown) may be formed on the substrate 202 prior to theformation of the first epitaxial layer 302. The nucleation layer maycomprise GaN or AlN. The first epitaxial layer 302 may comprise anyGroup III-V semiconductors, including binary, ternary, and quaternaryalloys of gallium, aluminum, indium, and nitrogen, also referred to asIII-nitride materials. In an example, the first epitaxial layer 302 maycomprise GaN. The first epitaxial layer 302 may be doped with n-typedopants, such as Si or Ge.

The epitaxial reflector 304 may be formed on the first epitaxial layer302. The epitaxial reflector 304 may be formed using the conventionalepitaxial techniques described above.

The epitaxial reflector 304 may comprise multiple layers of Group III-Vsemiconductor materials having different compositions. The epitaxialreflector 304 may comprise repetitions of a first layer 306 and a secondlayer 308. In an example, the first layer 306 may comprise AlInN and thesecond layer 308 may comprise GaN. The first layer 306 may have aconcentration of Al_(0.82)In_(0.18)N. The first layer 306 may have athickness of approximately 42 nm and the second layer 308 may have athickness of approximately 55 nm. The first layer 306 and the secondlayer 308 may have different refractive indices and thicknessesoptimized to maximize reflectivity at the wavelength and main emissionangle of the EL emission. The epitaxial reflector 304 may compriseapproximately 35 repetitions of the first layer 306 and the second layer308. The refractive index of the first layer 306 and the second 308layer may differ in their as-grown state due to differences in their inalloy composition or doping concentration. The additional epitaxiallayers may not form a good reflector in the as-grown state, but may begrown with differences in doping and/or alloy composition that affecttheir chemical reactivity in post-growth processing. Postgrowth-processing may transform the layers into an effective reflector,as described below.

In another example, the first layer 306 may comprise AlGaIn and thesecond layer 308 may comprise AlGaN. In this example, the first layer306 may have a concentration of Al_(0.80)Ga_(0.03)In_(0.17) and thesecond layer 308 may have a composition of Al_(0.02)Ga_(0.98)N.

The epitaxial reflector 304 may be made from any two layers ofAl_(x)In_(y)Ga_(z)N, where at least one of x, y, or z takes a differentvalue in each layer. Optimal values of x, y, and z may require fewerlayer repetitions and may have a better reflectivity characteristic vs.incident angle. Less desirable values of x, y, and z may still providesufficient reflectivity, but larger numbers of layer repetitions may berequired and angular characteristics may not be as preferable. Usefulranges of values for x, y, and z may be determined by requirements ofcrystal strain (e.g., a lattice parameter may not differ too much fromGaN's lattice parameter) and transparency (e.g., the band gap energy ofthe material may be larger than the photon energy of the EL emission).

The epitaxial reflector 304 may also comprise two layers of GaN withlarge differences in dopant concentration. For example, the first layer306 may be doped with Ge at a concentration of 10¹⁸ atoms/cm³ and thesecond layer 308 may be doped with Ge at a concentration of 10²⁰atoms/cm³. This combination may be used with or without a porosifyingreaction described below with reference to FIG. 6C. If the porosifyingreaction is not used, a large number of repetitions of the layers may beneeded (e.g., 100 repetitions).

The epitaxial reflector 304 may comprise alternating layers of an AlInNlayer.

A second epitaxial layer 310 may be formed on the epitaxial reflector304. The second epitaxial layer 310 may be formed using one or more ofthe epitaxial growth techniques described above. The second epitaxiallayer 310 may comprise any Group III-V semiconductors, including binary,ternary, and quaternary alloys of gallium, aluminum, indium, andnitrogen, also referred to as III-nitride materials. In an example, thesecond epitaxial layer 310 may comprise GaN. The second epitaxial layer310 may be doped with n-type dopants, such as Si or Ge. The secondepitaxial layer 310 may have the same dopant concentration as the firstepitaxial layer 302, or the dopant concentrations may different.

The n-type layer 204, the PL QW 206, the EL QW 210, and the p-type layer208 may be formed as described above with reference to FIGS. 2A-2C.

Referring now to FIGS. 4A-4F, cross-section views illustrating forming amicro-LED 400 incorporating the epitaxial reflector 304 are shown. Asshown in FIG. 4A, the epitaxial layers shown in FIG. 3 may be patternedand etched using conventional techniques to expose an upper surface ofthe n-type layer 204.

In FIG. 4B, a reflecting p-electrode 402 may be formed on an uppersurface of the p-type layer 208. The reflecting p-electrode 402 maycomprise any conductive material that reflects visible and/orultraviolet light, such as, for example, a refractive metal. Thereflecting p-electrode 402 may comprise one or more of a metal such assilver, a metal stack, a sequence of transparent conducting oxide layerswith different refractive indices, a series of dielectric layers withdifferent refractive indices on top of a transparent conductive oxidelayer, or combinations thereof. The reflecting p-electrode 402 may beformed using a conventional deposition technique, such as, for example,chemical vapor deposition (CVD), plasma-enhanced CVD (PECVD), atomiclayer deposition (ALD), evaporation, sputtering, chemical solutiondeposition, spin-on deposition, or other like processes.

In FIG. 4C, a conformal dielectric passivation layer 404 may be formedon the upper surface and sides of the p-type layer 208, sides of the ELQW 210, sides of the portion of the n-type layer 204, sides of the PL QW206, and an upper surface of the n-type layer 204. The dielectricpassivation layer 404 may comprise materials such as, but not limitedto, SiO₂ or SiN_(x).

In FIG. 4D, a reflecting n-type electrode 406 may be conformallydeposited on the dielectric passivation layer 404. The reflecting n-typeelectrode 406 may comprise any conductive material that reflects visibleand/or ultraviolet light, such as, for example, a refractive metal. Thereflecting n-type electrode 406 may comprise one or more of a metal suchas silver, a metal stack, a sequence of transparent conducting oxidelayers with different refractive indices, a series of dielectric layerswith different refractive indices on top of a transparent conductiveoxide layer, or combinations thereof. The reflecting n-type electrode406 may be formed using a conventional deposition technique, such as,for example, CVD, PECVD, ALD, evaporation, sputtering, chemical solutiondeposition, spin-on deposition, or other like processes.

The epitaxial reflector 304 may be electrically conducting and may beplaced within a distance of less than 1 micron from the reflectingp-type electrode 402 such that the epitaxial reflector 304 andreflecting p-type electrode 402 form an optical micro-cavity whichcontains both the PL and EL emitting QWs. The position of the ELemitting QWs within the micro-cavity may be selected to optimize theangular distribution of emitted EL radiation. The radiation distributionmay be controlled such that all or at least most of the EL is emittedinto angles for which the epitaxial reflector has a high reflectivity.The epitaxial reflector 304 may comprise a sequence of doped AlInN/GaNlayers or a sequence of porous GaN/non-porous GaN layers.

FIGS. 4E-4F illustrate the reflectivity of the epitaxial reflector 304based on wavelength and incident angle are shown. To improve wavelengthselectivity the epitaxial reflector 304 may take advantage of the higherrefractive index contrast between GaN and AlN at shorter vs. longerwavelengths. The epitaxial reflector 304 may also exploit differences inthe internal angular radiation profiles of PL QWs and EL QWs. The EL QWsmay be tailored to a desired radiation profile by changing the thicknessof the p-type layer 208. The radiation profile of the PL QWs fartheraway from the reflecting p-type electrode 402 may be less sensitive tothe thickness of the p-type layer 308.

As shown in FIG. 4E, light A emitted from the EL QW 210 may pass throughthe n-type layer 204, the PL QW 206, and the second epitaxial layer 310and may be reflected back by the epitaxial reflector 304.

As shown in FIG. 4F, light A emitted from the EL QW 210 may pass throughthe n-type layer 204, the PL QW 206, and the second epitaxial layer 310and may be reflected back by the epitaxial reflector 304. Light Bemitted from the PL QW 206 may pass through the n-type layer 204, thesecond epitaxial layer 310, the epitaxial reflector 304, the firstepitaxial layer 302, and the substrate 202 to exit the device.

Designs using the as-grown epitaxial reflector 304 may be most effectivein LEDs with small aspect ratios and reflecting sidewalls (e.g., thereflecting n-type electrode 406. Even when the micro-cavity is notformed, the EL emission may be incident on the epitaxial reflector 304over a relatively narrow range of angles and its reflectivity at largeangles of incidence may not be important. It should be noted the bottomsurface of the substrate 202 may be roughened or patterned to improveextraction of green PL emission that passes through the epitaxialreflector 304.

Referring now to FIG. 5, a chart illustrating reflectivity of theepitaxial reflector 304 over different angles of incidence for differentwavelengths of light is shown.

The wafer fabrication processing may be similar as that of aconventional single-wavelength LED and may include conventional stepssuch as chemical cleaning, acceptor activation anneal, dry etching ofmesas, and deposition of metal contacts, passivation and isolationlayers. A deeper mesa etch may be required due to the increasedthickness of epitaxial material between the p-type layer 208 and thehighly doped n-type layer 204 due to the addition of PL emitting QWsthat are not part of a standard LED structure.

The deposition of the reflecting p-type electrode 402 on the p-typelayer 208 may be similar as in a conventional LED process flow. Forexample, as described above, an opaque metal that has a highreflectivity across the visible and near-ultraviolet spectrum (e.g., Ag)may be evaporated or sputtered onto the surface of p-type layer 208.

The process flow described above may also be applied in a transparentLED process that uses a conducting oxide layer such as ITO as the p-typeelectrode 402. A dichroic mirror may be coated on top of the conductingoxide layer to make the electrode reflective at the EL emissionwavelength. The dichroic mirror may be applied in a transparent LED chipif the EL wavelength is around 400 nm or shorter. Small openings may bemade in the dichroic mirror around the edge of the chip to allow metalcontact to the ITO layer, which serves as the p-type electrode 402.

In addition, also applicable to transparent LEDs, the p-type electrode402 may be comprised of a multiple ITO layers with different porositythat result in differences in refractive index. This sequence may beobtained, for example, using sputter deposition with two different ITOtargets. One of the ITO targets may be oriented at an oblique anglerelative to the substrate 202. The thickness of a more porous ITO layer(i.e., lower refractive index) and a less porous ITO layer (i.e., higherrefractive index) may make an electrically conducting dichroic mirrorwith high reflectivity at the EL wavelength.

Referring now to FIGS. 6A-6C, cross-section views illustrating the useof an electrochemical reaction used to improve the reflectivity of theepitaxial reflector 304 are shown. The electrochemical reaction may beused in a subsequent post-growth processing step to selectivelytransform some of the epitaxial layers into a material with a differenteffective refractive index. This may produce a Bragg mirror with maximumreflectivity at the wavelength and main emission angle of the ELemission.

As shown in FIG. 6A, an etching process may be performed on thestructure illustrated in FIG. 4A to remove a portion of the n-type layer204, a portion of the second epitaxial layer 310, and a portion of theepitaxial reflector 304. In an example, a conventional dry etchingprocess may be used. The etching process may expose sidewalls of theepitaxial reflector 304, which may allow the composition of theepitaxial reflector 304 to be modified by the electrochemical reaction.The sidewalls of the epitaxial reflector 304 may be perpendicular to thewafer growth surface.

As described above, the first epitaxial layer 302 and the secondepitaxial layer 310 may both be doped with an n-type material, but mayhave different dopant concentrations. In an example, the first epitaxiallayer 302 may have a dopant concentration at its upper surface that isless than the dopant concentration of the second epitaxial layer 310.The lightly doped upper surface of the first epitaxial layer 302 mayserve as an etch stop during the etching process. The first epitaxiallayer 302 may have a higher dopant concentration in areas closer to thesubstrate 202 to allow for lateral electric conductivity.

The etching process may be the same one used to define the mesa shown inFIG. 4A. In other cases, an additional etching process may be performedafter removal of the substrate 202 as part of a vertical flip-chipprocess. The additional etching process may be used if the LED'sdimensions are large relative to the lateral diffusion distances in theelectrochemical reaction which are typically less than 100 microns.

After the dry etch exposes the upper surface of the first epitaxiallayer 302, a metal contact 602 may be formed on the first epitaxiallayer 302 using one or more of the deposition techniques describedabove. The metal contact 602 may enable the flow of current laterallythrough the doped regions of the first epitaxial layer 302 andsubsequently through the epitaxial reflector 304 and other doped layers.

The metal contact 602 may be coupled to one terminal of a power supply608 and the structure may be immersed in an electrolyte 602. In anexample, the electrolyte may be an acidic solution. Another terminal ofthe power supply 608 may be connected to a platinum foil counterelectrode 606, which may be immersed in the electrolyte 602 to completea circuit. Energizing the circuit may cause an electrochemical reactionthat selectively introduces microscopic voids (i.e., porosity) intolayers that are highly doped with Si or Ge. The porosity may decreasethe effective refractive index of these layers. This type of reactionmay produce epitaxial reflectors with high electrical conductivity andmay be preferable due to its relative simplicity of processing and itsapplicability to any structure, including micro-cavity LEDs that placethe conducting epitaxial reflector 304 in close proximity to the QWs.This reaction is illustrated in FIG. 6B.

In this type of reaction, the structure and the platinum foil counterelectrode 606 may be immersed in an electrolyte 602 of a 15M nitric acidsolution. A direct current may be applied through the platinum foilcounter electrode 606 and the metal contact 602, for example at acurrent density between 10 and 20 mA/cm². Optional ultra-violet (UV)illumination may be supplied by a 250 W mercury lamp. Depending on thelateral dimension of the dry etch pattern, processing times of 10 to 60minutes may be required after which the lamp and the current source areswitched off. Platinum may be applied directly over the surface of thestructure to make an electrical contact to the semiconductor surface andvarious different solutions such as NaOH, KOH, oxalic acid,nitrilotriacetic acid, or CH₃OH—HF—H₂O₂ may be used in theelectrochemical or photo-electrochemical process.

Another type of reaction may selectively oxidize nitride layers ofhigher Al mole fraction and may convert them into oxide or oxy-nitridelayers of lower refractive index than the as-grown material. In anexample, the electrolyte 602 used may be a basic solution. This reactionis illustrated in FIG. 6C.

This type of reaction may be suitable in implementations whereelectrical conductivity of the reflector is not important. Due to thelimited electrical conductivity of oxy-nitride materials, this approachmay require bonding the p-type layer 208 to a carrier, removing thesubstrate 202 and dry etching trenches through the first epitaxial layer302 from the back side to expose the epitaxial reflector 304. The backside of the wafer would then be subjected to an electrochemical process.A second trench may be etched from the opposite side of the structure toaccess electrically conducting GaN layers positioned in between theepitaxial reflector 304 and the p-type electrode 402.

A reaction that causes roughening of the nitrogen-polarity crystalsurfaces parallel to the substrate 202 that is exposed to theelectrolyte may occur simultaneously with the reactions described abovethat occurs on the perpendicular crystal surfaces. Alternatively,roughening of the nitrogen polarity crystal surfaces to improve lightextraction may be achieved in a separate processing step.

A transparent growth substrate and a dichroic mirror coating may be usedto make a green LED device without the need of an epitaxial reflector.This method may be preferred for its simplicity in LED designs as all orpart of the transparent growth substrate may remain attached in thefinished product.

Referring now to FIGS. 7A-7E, cross section views illustrating formingan LED device are shown. FIG. 7A shows forming trenches 702 in the greenLED utilizing PL of FIG. 2B is shown. As described above, the substrate202 may be a transparent growth substrate and may be a patternedsapphire substrate. As described above, a nucleation layer (not shown)may be formed on the patterned side of substrate 202 prior to theformation of the n-type layer 204. The nucleation layer may comprise GaNor AlN. The side of the substrate 202 opposite of the patterned side maybe grinded and polished to an optically smooth surface or surfaces.

The trenches 702 may be formed using a conventional directional etchingprocess, such as dry etching. The trenches may extend through an entirethickness of the p-type layer 208, an entire thickness of the EL QW 210,an entire thickness of the underlying portion of the n-type layer 204between the EL QW 210 and the PL QW 206, an entire thickness of the PLQW 206, and portion of the underlying n-type layer 204. The trenches 702may define an emitting area 712.

FIG. 7B shows forming a conformal dielectric passivation layer 704 inthe trenches 702 and on the p-type layer 208. The dielectric passivationlayer 704 may be formed using a conventional deposition technique, suchas, for example, CVD, PECVD, ALD, evaporation, sputtering, chemicalsolution deposition, spin-on deposition, or other like processes. Thedielectric passivation layer 704 may comprise materials such as but notlimited to SiO₂ or SiN_(x)

FIG. 7C shows forming n-type contacts 706 in the trenches 702 and ap-type contact 708 on the p-type layer 208. Portions of the dielectricpassivation layer 704 may be removed from a bottom of the trenches 702and the p-type layer 208 to expose the n-type layer 204 and the p-typelayer 208. The portions of the dielectric passivation layer 704 may beremoved using a conventional directional etching process, such as dryetching. The n-type contacts 706 and the p-type contact 708 may compriseone or more of a metal such as aluminum, silver, a metal stack, asequence of transparent conducting oxide layers with differentrefractive indices, a series of dielectric layers with differentrefractive indices on top of a transparent conductive oxide layer, orcombinations thereof. The n-type contacts 706 and the p-type contact 708may be formed using a conventional deposition technique, such as, forexample, CVD, PECVD, ALD, evaporation, sputtering, chemical solutiondeposition, spin-on deposition, or other like processes.

FIG. 7D shows forming a dichroic mirror 710 a bottom surface of thesubstrate 202 to form the LED device. The dichroic mirror 710 maycomprise a stack of dielectric layers with a large difference inrefractive index. For example, the dichroic mirror 710 may comprise SiO₂and one or more of TiO₂, ZrO₂, and HfO₂. The dichroic mirror 710 may beformed using one or more conventional techniques, such as atomic layerepitaxy, sputtering or e-beam evaporation.

The dichroic mirror 710 may have a thickness that is ¼ the peakwavelength of the shorter wavelength EL, but more complex coatingdesigns are possible. The optimum dichroic mirror design for a specificdevice may depend on factors such as the geometry of the patternedsapphire and the amount of shorter-wavelength light permitted to escapefrom the device per application requirements. For white LEDapplications, the reflectivity of the dichroic mirror may beintentionally designed to be much lower than 100% to allow enough blueEL emission to escape to produce white light. As shown in FIG. 7D, lightA emitted from the EL QW 210 may pass through the n-type layer 204, thePL QW 206, and the substrate 202 and may be reflected back by thedichroic mirror 710. The reflected light A may then enter the PL QW 206and be emitted as light B, which may pass through the n-type layer 204,the substrate 202, and the dichroic mirror 710.

FIG. 7E, shows another example of forming the dichroic mirror 710 suchthat it extends to sides of the LED device. The dichroic mirror 710 maybe formed using one or more conventional conformal depositiontechniques, such as atomic layer epitaxy, sputtering or e-beamevaporation. In addition to the bottom of the substrate 202, thedichroic mirror may be formed on one or more of: sidewalls of the p-typelayer 208, sidewalls of the EL QW 210, sidewalls of the n-type layer204, sidewalls of the PL QW 206, and sidewalls of the substrate 202.

Referring now to FIGS. 8A-8E, cross section views illustrating formingan LED device are shown. FIG. 8A shows forming trenches 802 in the greenLED utilizing PL of FIG. 2B is shown. As described above, the substrate202 may be a transparent growth substrate and may be a patternedsapphire substrate. As described above, a nucleation layer (not shown)may be formed on the patterned side of substrate 202 prior to theformation of the n-type layer 204. The nucleation layer may comprise GaNor AlN. The side of the substrate 202 opposite of the patterned side maybe grinded and polished to an optically smooth surface or surfaces.

The trenches 802 may be formed using a conventional directional etchingprocess, such as dry etching. The trenches may extend through an entirethickness of the p-type layer 208 and an entire thickness of the EL QW210, and may stop in the portion of the n-type layer 204 between the ELQW 210 and the PL QW 206. The trenches 802 may define an emitting area812.

FIG. 8B shows forming a conformal dielectric passivation layer 804 inthe trenches 802 and on the p-type layer 208. The dielectric passivationlayer 804 may be formed using a conventional deposition technique, suchas, for example, CVD, PECVD, ALD, evaporation, sputtering, chemicalsolution deposition, spin-on deposition, or other like processes. Thedielectric passivation layer 804 may comprise materials such as but notlimited to SiO₂ or SiN_(x).

FIG. 8C shows forming n-type contacts 806 in the trenches 802 and ap-type contact 808 on the p-type layer 208. Portions of the dielectricpassivation layer 804 may be removed from a bottom of the trenches 802and the p-type layer 208 to expose the n-type layer 204 and the p-typelayer 208. The portions of the dielectric passivation layer 804 may beremoved using a conventional directional etching process, such as dryetching. The n-type contacts 806 and the p-type contact 808 may compriseone or more of a metal such as aluminum, silver, a metal stack, asequence of transparent conducting oxide layers with differentrefractive indices, a series of dielectric layers with differentrefractive indices on top of a transparent conductive oxide layer, orcombinations thereof. The n-type contacts 806 and the p-type contact 808may be formed using a conventional deposition technique, such as, forexample, CVD, PECVD, ALD, evaporation, sputtering, chemical solutiondeposition, spin-on deposition, or other like processes.

FIG. 8D shows forming a dichroic mirror 810 on a bottom surface of thesubstrate 202 to form the LED device. The dichroic mirror 810 maycomprise a stack of dielectric layers with a large difference inrefractive index. For example, the dichroic mirror 810 may comprise SiO₂and one or more of TiO₂, ZrO₂, and HfO₂. The dichroic mirror 810 may beformed using one or more conventional techniques, such as atomic layerepitaxy, sputtering or e-beam evaporation.

The dichroic mirror 810 may have a thickness that is ¼ the peakwavelength of the shorter wavelength EL, but more complex coatingdesigns are possible. The optimum dichroic mirror design for a specificdevice may depend on factors such as the geometry of the patternedsapphire and the amount of shorter-wavelength light permitted to escapefrom the device per application requirements. For white LEDapplications, the reflectivity of the dichroic mirror may beintentionally designed to be much lower than 100% to allow enough blueEL emission to escape to produce white light. As shown in FIG. 8D, lightA emitted from the EL QW 210 may pass through the n-type layer 204, thePL QW 206, and the substrate 202 and may be reflected back by thedichroic mirror 810. The reflected light A may then enter the PL QW 206and be emitted as light B, which may pass through the n-type layer 204,the substrate 202, and the dichroic mirror 810.

FIG. 8E, shows another example of forming the dichroic mirror 810 suchthat it extends to sides of the LED device. The dichroic mirror 810 maybe formed using one or more conventional conformal depositiontechniques, such as atomic layer epitaxy, sputtering or e-beamevaporation. In addition to the bottom of the substrate 202, thedichroic mirror may be formed on one or more of: sidewalls of the p-typelayer 208, sidewalls of the EL QW 210, sidewalls of the n-type layer204, sidewalls of the PL QW 206, and sidewalls of the substrate 202.

Referring now to FIGS. 9A-9B, charts illustrating reflectivity of theLED device of FIGS. 7A-8D over different angles of incidence fordifferent wavelengths of light are shown. FIG. 9A shows reflectivityfrom the substrate 202 side of the LED with a dichroic mirror 710comprising 12 repetitions of a 51 nm thick layer of ZrO₂ and a 67 nmthick layer of SiO₂ as well as a 90 nm thick layer of ZrO₂. FIG. 9Bshows reflectivity from the substrate 202 side of the LED with adichroic mirror 710 comprising 4 repetitions of a 53 nm thick layer ofZrO₂ and a 69 nm thick layer of SiO₂ as well as a 60 nm thick layer ofZrO₂. The dichroic mirror 710 may maintain higher reflectivity over awider range of angles for transverse-electric (TE) polarized light. Thereflectivity responses in FIGS. 9A-9B are shown for the case of 90% TEpolarized light.

Referring now to FIG. 10, a chart illustrating power emitted as afunction of distance from the p-type contact 608 is shown. Morespecifically, FIG. 10 shows the calculated fraction of totalelectroluminescent power emitted with TE polarization for a 400 nmemitting QW as a function of its distance from a silver p-type contact608. The thickness of the p-type layer 208 may be optimized to exploitself-interference of the EL emitting QWs. At the wavelength of 400 nmthe fraction of EL power emitted with TE polarization may be maximizedwhen the QWs are placed approximately at a distance equivalent toapproximately 0.25 to approximately 0.45 times the peak wavelength ofthe EL emission in a p-type GaN (not the vacuum wavelength) from thep-type contact 608.

As described above, these methods and apparatuses may improve thewall-plug efficiency of high-power LEDs with applications including butnot limited to micro-LED displays. The improvements may be mostpronounced at higher current densities and longer wavelengths. The abovedescription focuses mainly on green LEDs, but the techniques describedabove may be applied to blue LEDs (i.e., blue PL emitting QWs that arepumped by shorter WL EL emitting QWs) when driving current densities aresufficiently high. The techniques described above may be advantageousfor direct color LEDs in which the color sensitivity to changes ininjection current needs to be minimized. The color may be stable over awide range of luminance levels. The techniques described above may beused in white LEDs with small form factors that do not include anexternal phosphor conversion material. These LEDs may find applicationsin smart automotive headlights and other products that use beam-steeringtechnology based on LED arrays.

Although features and elements are described above in particularcombinations, one of ordinary skill in the art will appreciate that eachfeature or element can be used alone or in any combination with theother features and elements. In addition, the methods described hereinmay be implemented in a computer program, software, or firmwareincorporated in a computer-readable medium for execution by a computeror processor. Examples of computer-readable media include electronicsignals (transmitted over wired or wireless connections) andcomputer-readable storage media. Examples of computer-readable storagemedia include, but are not limited to, a read only memory (ROM), arandom access memory (RAM), a register, cache memory, semiconductormemory devices, magnetic media such as internal hard disks and removabledisks, magneto-optical media, and optical media such as CD-ROM disks,and digital versatile disks (DVDs).

1. A light emitting diode (LED) device comprising: atransparent-substrate; an n-type layer disposed on or above thetransparent substrate; an electroluminescent (EL) quantum well (QW)formed on the n-type layer and configured to emit a first light, aphotoluminescent (PL) QW formed in the n-type layer and configured toabsorb at least a portion of the first light and in response emit asecond light having a longer wavelength than the first light, the EL QWand the PL QW separated from one another by a portion of the n-typelayer; a p-type layer formed on the EL QW; and a dichroic reflectorhaving a greater reflectivity for the first light than for the secondlight, wherein the dichroic reflector is arranged to transmit the secondlight out of the LED device and to reflect back to the PL QW a portionof the first light transmitted through the PL QW and incident on thedichroic reflector.
 2. The LED device of claim 21, wherein the dichroicreflector extends along one or more sidewalls of the transparentsubstrate, n-type layer, EL QW, and p-type layer.
 3. The LED device ofclaim 1, wherein the PL QW comprises multiple QWs emitting a samewavelength of light.
 4. The LED device of claim 1, wherein the EL QWcomprises multiple QWs emitting a same wavelength of light.
 5. The LEDdevice of claim 1, wherein the PL QW is adjacent to a depletion regionof a p-n junction between the n-type layer and the p-type layer.
 6. TheLED device of claim 1, wherein the thickness of the p-type layer isoptimized to exploit self-interference of the EL QW.
 7. The LED deviceof claim 1, wherein the EL QW and the p-type electrode are separated bya distance equivalent to approximately 0.25 to approximately 0.45 timesa peak wavelength of an emission of the EL QW in the p-type layer. 8.The LED device of claim 21, wherein the dichroic reflector comprises astack of dielectric layers having different refractive indices.
 9. TheLED device of claim 1, wherein the dichroic reflector has a thickness ofapproximately one quarter of a peak wavelength of light emitted by theEL QW. 10-12. (canceled)
 13. The LED device of claim 1, wherein EL QW islocated within a depletion region of a p-n junction between the n-typelayer and the p-type layer. 14-16. (canceled)
 17. The LED device ofclaim 24, wherein one or more of the multiple layers of Group III-Vsemiconductor materials are oxidized such that they have a lowerrefractive index than as-grown material.
 18. The LED device of claim 24,wherein one or more of the multiple layers of Group III-V semiconductormaterials comprise dopants of one or more of Si and Ge and are processedto include voids such that they have a lower refractive index thanas-grown material. 19-20. (canceled)
 21. The LED device of claim 1,wherein the dichroic reflector is disposed on a surface of thetransparent substrate opposite from the n-type layer.
 22. The LED deviceof claim 21, further comprising trenches formed through at least anentire thickness of the p-type layer and an entire thickness of the ELQW to expose the n-type layer, the trenches defining an emitting area; apassivation material formed on sidewalls of the trenches and an uppersurface of the p-type layer; n-type contacts formed in the trenches; anda p-type contact formed on the upper surface of the p-type layer in theemitting area.
 23. The LED device of claim 1, wherein the dichroicreflector is disposed between the transparent substrate and the n-typelayer.
 24. The LED device of claim 23, wherein the dichroic reflectorcomprises multiple layers of Group III-V semiconductor materials havingdifferent compositions.
 25. The LED device of claim 24, furthercomprising: a first epitaxial layer formed on the substrate between thesubstrate and the dichroic reflector; and a second epitaxial layerformed between the dichroic reflector and the n-type layer.